In an optical communications field, wavelength-division multiplexing (WDM) transmission systems have been studied for enhancing information capacity. In these systems, a plurality of optical signals at different wavelengths which are spaced by about 1 nm are transmitted over a single optical fiber. For this purpose, an optical wavelength multiplexer/demultiplexer is an important component which plays a significant roll to combine or separate the optical signals at different wavelengths. Particularly, one of the most promising optical wavelength multiplexer/demultiplexers for such use is one which utilizes an arrayed-waveguide grating, which may increase the number of multiplexed signals with relatively narrow wavelength spacings.
One of conventional optical wavelength multiplexer/demultiplexers is disclosed in Japanese Published Patent Application No. 4-163406.
The conventional optical wavelength multiplexer/demultiplexer comprises at least one input channel waveguide for receiving wavelength division multiplexed signals, the wavelength division multiplexed signal comprising a plurality of signals having a predetermined wavelength difference from each other, an input slab waveguide for expanding the wavelength division multiplexed signals coupled from the input channel waveguide into the input slab waveguide, an arrayed-waveguide grating comprising a plurality of channel waveguides, each channel waveguide having a predetermined length difference in accordance with the predetermined wavelength difference, so that each signal at different wavelength coupled to and traveling over each channel waveguide is provided with a phase difference from each other in accordance with the predetermined length difference, an output slab waveguide for focusing the signals at different wavelength coupled from the channel waveguides into a plurality of predetermined positions in accordance with the predetermined wavelength difference, respectively, and a plurality of output channel waveguides, an input end of each output channel waveguide being arranged at each predetermined position, so that each separated signal at each wavelength is coupled to each output channel waveguide and emerges from an output end thereof.
In operation, the wavelength division multiplexed signals coupled into the input channel waveguide, expand into the input slab waveguide by diffraction. Then, the expanded signals are distributed to the channel waveguides of the arrayed-waveguide grating, which are arranged radially along an arc boundary of the input slab waveguide. On the other hand, as each channel waveguide of the arrayed-waveguide grating has a predetermined waveguide length difference, each signal, after traveling over each channel waveguide to the output slab waveguide, has a predetermined phase difference according to its waveguide length difference. Since the phase difference depends on the wavelength of the signal, each signal at different wavelength is focused on a different position along the arc boundary of the output slab waveguide due to a lens effect. As a result, separated signals each having a different wavelength are received by the plurality of output channel waveguides and emerge therefrom, respectively.
In the conventional optical wavelength multiplexer/demultiplexer, however, there is a disadvantage in that each separated signal emerging from each output channel waveguide suffers a different loss from others in a relatively large amount when they are separated. That is to say, as the output channel waveguide is arranged at a position which is more distant from a symmetrical axis of the output slab waveguide, the loss for the signal emerging therefrom becomes higher. It is important for an optical wavelength multiplexer/demultiplexer to attain an uniform overall transmission loss for each separated signal at different wavelength.